Variable length transmission line

ABSTRACT

A pair of movable U-shaped inner conductors for portions of a transmission line each have yoke elements secured to a slide positioned in a chamber in a metal housing and parallel leg elements slidably received in the ends of corresponding stationary tubular conductors also positioned in the housing and forming inner conductors of other portions of the transmission line. Two of the stationary conductors have other ends terminating in external connectors at one end of the housing and a third stationary conductor is U-shaped to provide a continuous inner conductor between the conductors. The casing is the outer conductor for the transmission line and has inner conducting wall surfaces spaced predetermined distances from the different portions of inner conductor to provide a substantially uniform characteristic impedance along the transmission line. A rotatable worm member journaled in the casing and threaded through the slide is rotatable to move the slides and U-shaped movable conductor members relative to the housing and stationary conductor members to vary the length of the transmission line.

United States Patent [191 Hannaford et al.

[ VARIABLE LENGTH TRANSMISSION LINE [751 Inventors: David A.I-Iannaford, Beaverton;

Cornelis T. Veenendaal, Milwaukie, both of Oreg.

[73] Assignee: Tektronix, Inc., Beaverton, Oreg.

[22] Filed: Mar. 8, I971 [21] Appl. No.: 121,807

[52] US. Cl. .1 333/31 R, 333/33 [51] Int. Cl. H0lp 9/00, H01p 1/00 [58]Field of Search 333/31 R, 97 R, 96, 333/84 M [56] References CitedUNITED STATES PATENTS 2,440,089 4/1948 Haeff 333/31 R 2,721,312 10/1955Grieg et al. 333/84 M 3,146,413 8/1964 Butler 333/31 R 2,502,359 3/19501 Wheeler 333/31 R 2,623,122 12/1952 Weber et al. 333/96 2,961,62311/1960 Kuecken et al., 333/84 M X 3,005,168 10/1961 Fye 333/31 R3,114,887 12/1963 Wedemeyer 333/84 M X 3,309,629 3/1967 Hessler, Jr.383/97 R X 3,354,411 11/1967 Selby 333/97 R X 3,516,028 6/1970 Leef etal 3.33/31 R FOREIGN PATENTS OR APPLICATIONS 924,156 7/1947 France333/31 R 833,511 4/1960 Great Britain 333/31 R OTHER PUBLICATIONS King,D. D. Measurements at Centimeter Wavelength," D. Van Nostrand Co., 1952pp. 40-47. McAlister, K. R. A Variable-Length Radio FrequencyTransmission-Line Section, Jr. of Scientific Instruments, Vol. 28,5-1951 pp. 142-143.

Adey et al., A'Microwave Waveguide Trombone Phase Shifter, Canadian Jr.of Physics, Vol. 34, 1956, pp. 1112-1114.

I-Ioffswell, R. A., A Microstrip Line Stretcher," Review of ScientificInstruments, Vol. 41 -9-1970, pp. 1330-1331.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. I-I. PunterAttorney-Buckhorn, Blore, Klarquist & Sparkman [57] ABSTRACT A pair ofmovable U-shaped inner conductors for portions of a transmission lineeach have yoke elements secured to a slide positioned in a chamber in ametal housing and parallel leg elements slidably received in the ends ofcorresponding stationary tubular conductors also positioned in thehousing and forming inner conductors of other portions of thetransmission line. Two of the stationary conductors have other endsterminating in external connectors at one end of the housing and a thirdstationary conductor is U-shaped to provide a continuous inner conductorbetween the conductors. The casing is the outer conductor for thetransmission line and has inner conducting wall surfaces spacedpredetermined distances from the different portions of inner conductorto provide a substantially uniform characteristic impedance along thetransmission line. A rotatable worm member journaled in the casing andthreaded through the slide is rotatable to move the slides and U-shapedmovable conductor members relative to the housing and stationaryconductor members to vary the length of the transmission line.

799.1% wr n fi ures,

Patented Oct. 2, 1973 2 Sheets-Sheet 1 ATTORNEYS Patented Oct. 2, 19733,763,445

2 Sheets-Sheet 2 Ai A DAVID A. HANNAFORD IO 28 CORNELIS VEENENDAALINVENTORS.

BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYS VARIABLE LENGTHTRANSMISSION LINE BACKGROUND OF THE INVENTION Prior attempts to providetransmission lines which can be varied in length while maintaining thecharacteristic impedance substantially uniformly along the line have, ingeneral, employed arrangements in which portions of both the inner andouter conductors of the lines slide relative to other portions of suchconductors. For example, one portion of the outer conductor of a coaxialline is slidable longitudinally into another portion of such outerconductor in telescoping engagement and simultaneously the correspondingportions of the inner conductor have similar telescoping relativesliding engagement. This type of structure is cumbersome and difiicultto fabricate since sliding joints must be provided for portions of bothof the conductors. Also each of the sliding joints is a potential sourceof trouble since it is essential that good electric contact between therelative movable portions of each conductor be maintained at the pointat which one portion of each conductor slidably enters the other portionof the corresponding conductor.

SUMMARY OF THE INVENTION In accordance with the present invention, aconducting housing, all portions of which are stationary, provides anouter conductor for a variable length transmission line and a portion ofthe inner conductor only is moved relatively to other portions of theinner conductor and the housing so that, in general, the number ofsliding joints, requiring the good electrical contact referred to above,is one half of that of prior structures. Also the movable portions ofthe inner conductor are positioned in a chamber in the housing whichprovides guide surfaces for slides supporting and protecting thismovable portion of the inner conductor and which can also housecalibrated mechanism for moving such movable portions while maintainingthe characteristic impedance substantially uniform along thetransmission line.

It is therefor an object of the invention to provide an improvedvariable length transmission line in which the length of the line isvaried by moving a portion of an inner conductor only relative toanother portion of such conductor in a housing providing an outerconductor for such line, while maintaining the characteristic impedanceof the transmission line substantially constant along such line.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a variablelength transmission line in accordance with the present invention withthe top cover mostly broken away and internal parts also broken away toshow internal structure;

FIG. 2 is a longitudinal vertical section taken on the line 2-2 of FIG.1;

FIG. 3 is a lateral vertical section taken on the line 3-3 of FIG. 2;

FIG. 4 is a view similar to FIG. 3, taken on the line 44 of FIG. 2;

FIG. 5 is a fragmentary vertical section on an enlarged scale taken onthe line 5-5 of FIG. 1;

FIG. 6 is a fragmentary side elevation on an-enlarged scale of a slidingjoint in the inner conductor of the transmission line;

FIG. 7 is a vertical section taken on the line 7-7 of FIG. 6;

FIG. 8 is a fragmentary vertical section on an en larged scale taken onthe line 88 of FIG. 1;

FIG. 9 is a vertical section taken on the line 9-9 of FIG. 8; and

FIG. 10 is a vertical section on an enlarged scale taken on the line 10-10 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The housing 10 of thetransmission line is shown in FIGS. 1 and 2, and includes a body member12 of rectangular shape in plan view provided with a cover member 14 ofthe same shape secured to the body member by screws 16. One end portion18 of the body member 12 is of greater thickness than the remainder ofsuch body member 12 and has a longitudinally extending rectangularchannel 20 in its upper surface adjacent each of its sides and a similarU-shaped centrally disposed channel 22 also in its upper surface. The U-shaped channel 22 has a curved portion 24 adjacent the left of the endportion 18 of the body member 12 in FIG. 1 which connects straightportions of the channel 22 extending parallel to each otherlongitudinally of the body member. The channels 20 also extend parallelto the straight legs of the channel 22 and terminate at their left endsin FIGS. 1 and 2 adjacent but spaced from the left end of the bodymember 12. A bore 26 shown in FIGS. 1 and 8 extends into the left end 18of the body member 12 in axial alignment with each of the channels 20.

The remaining thinner end portion 28 of the body member 12 has agenerally rectangular recess 30 which terminates at one of its endsadjacent the thicker end portion 18 of the body member. The remainder ofthe recess 30 is bounded by side wall portions 32 and an end wallportion 34 integral with the body member 12.

The cover member 14 has an end portion 36 covering and fitting the uppersurface of the end portion 18 of the body member 12 and an opposite endportion 38 covering the end portion 28 of the body member 12 andcovering and fitting the top surfaces of the side walls 32 and end wall34 of the body member 12. The upper surface of the end portion 28 of thebody member 12 within the recess 30 is a plane except for a centrallongitudinal groove 35 which is opposite a similar groove 3 5a (FIG. 4)in a similar lower surface of the end portion 38 of the cover member 14which is parallel and spaced from such upper surface to provide arectangular chamber in the housing 10 having smooth and parallel innertop and bottom wall surfaces and smooth and parallel side wall surfaces.

Stationary tubular portions 40 of the inner conductor of thetransmission line are supported in the channels 20 in the end portions18 of the body member 12 by low loss insulating elements 42. Two of suchelements 42 of generally rectangular cross section fit in each channel20 and have semicircular grooves in their adjacent side surfacesreceiving and fitting the tubular inner conductors 40. As shown in FIGS.1 and 8, these inner tubular conductors 40 are connected adjacent theleft end of the body member 12 in FIG. 1 to connectors 44 received inthe bores 26 in such body member. These connectors have externalconnector portions 46 for connection to flexible coaxial cables or thelike forming continuations of the transmission line of the presentinvention.

A stationary U-shaped tubular inner conductor 48 is similarly supportedin the U-shaped channel 22 by similar straight insulating elements 50and curved insulating elements 52. All of the tubular inner conductorshave open ends 54 projecting a short distance into the chamber formed bythe recess 30 in the body member 12. These ends 54 are slotted as shownmost clearly in FIG. 6 to provide axially extending spring fingers 56which are bent inwardly into resilient engagement with the straight legsof a pair of U-shaped slidable inner conductors 58 having curvedportions 60 positioned in the chamber formed by the recess 30 in thebody member 12. The legs of the conductors 58 are round in cross sectionand one leg of each of such conductors 58 is received in one of thestraight tubular inner conductors 40 and the other leg of each conductor58 is received in one of the legs of the U-shaped tubular innerconductor 48.

The curved ends 60 of the slidable U-shaped inner conductors 58 areflattened, as shown most clearly in FIG. 5, by removal of materialtherefrom and are each received in a semicircular groove 61 in eachslide member of a pair of slide members 64 and 66, both positioned inthe recess 30 in the body member 12. The slide members 64 and 66 eachhave a recess 68 (FIG. in their top portion for receiving a coverelement 70. The semicircular groove 61 is positioned in a plane surfaceforming the bottom of each recess 68 and a corresponding semicirculargroove is positioned in the lower surface of each cover element 70 tosecurely hold the curved portion 60 of the U-shaped sliding conductor 58in the respective slide member 64 or 66. Both the cover elements 70 andthe slide members 64 and 66 are made of low loss insulating material.

The slide members 64 and 66 are moved in the recess 30 in the bodymember 12 of the housing by a worm 72 positioned between these slidemembers and rotatable in bearing elements 74 mounted in suitablerecesses in the body member 12 and cover member 14 of the housing 10.The worm 72 extends parallel to the straight legs of the U-shapedsliding conductors 58 and as shown in FIG. 4, the grooves 35 in the bodymember 12 and cover member 14 of the housing 10 provide room for theworm. The sides of the slide members 64 and 66 adjacent the worm havethread elements 76 engaging the threaded portion of the worm. A driveshaft 78 projects from one end of the worm 72 through the end wall 34 ofthe body member 12 of the housing 10.

It will be apparent that any suitable mechanism such as a manuallyrotatable knob on the drive shaft 78 can be employed to rotate the worm72 to thereby move the slide members 64 and 66 to slide the straightlegs of the U-shaped sliding inner conductors 58 into and out of thetubular inner conductors 40 and the straight portions of the tubularU-shaped inner conductor 48. In a particular construction the range ofsliding movement of the U-shaped sliding conductors was approximately 3inches so that the change in length of the inner conductor of thetransmission line effected by moving this range was approximately 12inches. This change in length of the inner conductor effectively changesthe length of the transmission line and the latter change in length is alinear function of the rotation of the worm 72. The range of this changein length may be made greater or smaller by changing the lengths of therespective portions of the inner conductors and the range of theirrelative movement, or providing a greater or lesser number of U-shapedsliding inner conductors and cooperating outer conductors.

The variable length transmission line of the present invention willordinarily be connected by at least one of the connectors 44 to anexternal transmission line having a given characteristic impedance. Theconnectors 44 will be selected to have a substantially uniformcharacteristic impedance which is the same as that of the externaltransmission line. The housing 10 is made of metal or at least haselectrically conducting interior surfaces in' the channels 20 and 22 andthe chamber provided by the recess 30 in the body member 12. Thedimensions of the channels 20 and 22 containing the tubular innerconductors 40 and 48 are selected to provide, in conjunction with theouter diameter of the tubular conductors 40 and 48 and the dielectricconstant of the insulating elements 42 and 50, this same characteristicimpedance along the portions of the transmission line in which thetubular inner conductors are supported by such insulating elements.

The distance between the upper and lower plane surfaces in the interiorof the chamber provided by the recess 30 in the body member 12 is alsoselected so that this distance in conjunction with the diameter of theexposed leg portions of the U-shaped sliding conductors 58 provides thissame characteristic impedance. Since the curved portions 60 of thesesliding conductors have insulating portions of the slide members 64 and66 positioned between such curved portions and the conducting planesurfaces of the chamber in which they are positioned, these curvedportions 60 are reduced in thickness as shown in FIG. 5 to also causethe corresponding portion of the transmission line to have this samecharacteristic impedance.

There must necessarily be a change of diameter of the inner conductor ofthe variable length transmission line at each of the sliding jointsbetween the sliding inner conductors 58 and the stationary tubularconductors 40 and 48. As shown most clearly in FIG. 6, the ends 54 ofthese tubular conductors are tapered and the taper is selected andpositioned relative to the ends of the insulating elements 42 and 50 andthe change in spacing of the inner surfaces of the outer conductor atthe junction between the recess 30 in the body member 12 of the housing10 with the channels 20 and 22, including the small recesses 80 (FIGS. 1and 10) which are present adjacent this junction as a result of amachining operation producing the channels 20 and 22, to provide asubstantially uniform characteristic impedance along the transmissionline across this junction. This uniform characteristic impedancerequires that there be good electrical contact between the sliding innerconductors 58 and the tubular inner conductors 40 and 48 at the smallend of the taper portion 54 on the tubular inner conductors. This isprovided by the spring fingers 56 and by highly conductive corrosionresistance coatings, for example silver plating covered by a thin goldplating, on at least the portions of the conductors making the slidingjoints between the stationary and sliding inner conductors. The resultis an accurately variable length compact transmission line having asubstantially uniform characteristic impedance along its entire length.

We claim:

1. A variable length transmission line comprising:

stationary outer conductor means surrounding stationary hollow innerconductor means defining coaxial transmission line means extendingparallel to each other, adjacent outer ends of said stationary hollowinner conductor means being connected together by curved stationaryhollow inner conductor means;

movable inner conductor means having parallel leg means each having oneend received in sliding electrical relationship in one end of one ofsaid stationary hollow inner conductor means and having connectingsection means connecting the other ends of said leg means together;

' housing means having a recess therein with said movable innerconductor means being disposed therein and movable therea'long; saidrecess having conducting surface means provided by said housing meansextending along each side of said movable inner conductor means andbeing spaced from said movable inner conductor means thereby defining anouter conductive means therefor so that said coaxial transmission linemeans, said conducting surface means and said movable inner conductormeans provide said transmission line with substantially uniformcharacteristic impedance therealong;

slide means disposed in said chamber connected to said movable innerconductor means and being movable along said recess to move said movableinner conductor means relative to said housing means and said parallelleg means relative to said stationary hollow inner conductor means tovary the length of said transmission line; and b means connected to saidslide means for moving said slide means and said movable inner conductormeans along said recess.

2. A variable length transmission line comprising:

stationary outer conductor means surrounding stationary hollow innerconductor means defining coaxial transmission line means extendingparallel to each other;

movable inner conductor means having parallel leg means each having oneend received in sliding electrical relationship in one end of one ofsaid stationary hollow inner conductor means and having connectingsection means connecting the other ends of 45 and movable therealong;said recess having conducting surface means provided by said housingmeans extending along each side of said movable inner conductor meansand being spaced from said movable inner conductor means therebydefining an outer conductive means therefor so that said coaxialtransmission line means, said conducting surface means and said movableinner conductor means provide said transmission line with substantiallyuniform characteristic impedance therealong;

slide means disposed in said recess having insulation means connected tosaid connecting section means, said connecting section means being oflesser size than said leg means, said slide means being movable alongsaid recess to move said movable inner conductor means relative to saidhousing ine ans and said parallel leg means relative to said stationaryhollow inner conductor means to vary the length of said transmissionline; and

means connected to said slide means for moving said slide means and saidmovable inner conductor means along said.recess.

3. A variable length transmission line according t d claim 1 whereinsaid coaxial transmission line means is disposed in said housing meansand said conducting surfaces constitute said stationary outer conductormans;

4. Ayariable length transmission line according to" claim 1 wherein saidcoaxial transmission line means are in parallel.

of said movable inner conductor means, said slide means having threadareas threadably engaging said worm member.

TA variable length transmission line according to claim 3 wherein saidconducting surface means are positioned closer to said movable innerconductor means than said stationary outer conductor means arepositioned with respect to said stationary hollow inner conductor means.

7. A variable length transmission line according to claim 3 wherein saidconnecting section means has an arcuate configuration.

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1. A variable length transmission line comprising: stationary outerconductor means surrounding stationary hollow inner conductor meansdefining coaxial transmission line means extending parallel to eachother, adjacent outer ends of said stationary hollow inner conductormeans being connected together by curved stationary hollow innerconductor means; movable inner conductor means having parallel leg meanseach having one end received in sliding electrical relationship in oneend of one of said stationary hollow inner conductor means and havingconnecting section means connecting the other ends of said leg meanstogether; housing means having a recess therein with said movable innerconductor means being disposed therein and movable therealong; saidrecess having conducting surface means provided by said housing meansextending along each side of said movable inner conductor means andbeing spaced from said movable inner conductor means thereby defining anouter conductive means therefor so that said coaxial transmission linemeans, said conducting surface means and said movable inner conductormeans provide said transmission line with substantially uniformcharacteristic impedance therealong; slide means disposed in saidchamber connected to said movable inner conductor means and beingmovable along said recess to move said movable inner conductor meansrelative to said housing means and said parallel leg means relative tosaid stationary hollow inner conductor means to vary the length of saidtransmission line; and means to said slide means for moving said slidemeans and said movable inner conductor means along said recess.
 2. Avariable length transmission line comprising: stationary outer conductormeans surrounding stationary hollow inner conductor means definingcoaxial transmission line means extending parallel to each other;movable inner conductor means having parallel leg means each having oneend received in sliding electrical relationship in one end of one ofsaid stationary hollow inner conductor means and having connectingsection means connecting the other ends of said leg means together;housing means having a recess therein with said movable inner conductormeans being disposed therein and movable therealong; said recess havingconducting surface means provided by said housing means extending alongeach side of said movable inner conductor means and being spaced fromsaid movable inner conductor means thereby defining an outer conductivemeans therefor so that said coaxial transmission line means, saidconducting surface means and said movable inner conductor means providesaid transmission line with substantially uniform characteristicimpedance therealong; slide means disposed in said recess havinginsulation means connected to said connecting section means, saidconnecting section means being of lesser size than said leg means, saidslide means being movable along said recess to move said movable innerconductor means relative to said housing means and said parallel legmeans relative to said stationary hollow inner conductor means to varythe length of said transmission line; and means connected to said slidemeans for moving said slide means and said movable inner conductor meansalong said recess.
 3. A variable length transmission line according toclaim 1 wherein said coaxial transmission line means is disposed in saidhousing means and said conducting surfaces constitute said stationaryouter conductor means.
 4. A variable length transmission line accordingto claim 1 wherein said coaxial transmission line means are in parallel.5. A variable length transmission line according to claim 1 wherein saidmeans for moving said slide means includes a worm member journaled insaid housing and extending along said recess parallel to said leg meansof said movable inner conductor means, said slide means having threadareas threadably engaging said worm member.
 6. A variable lengthtransmissiOn line according to claim 1 wherein said conducting surfacemeans are positioned closer to said movable inner conductor means thansaid stationary outer conductor means are positioned with respect tosaid stationary hollow inner conductor means.
 7. A variable lengthtransmission line according to claim 1 wherein said connecting sectionmeans has an arcuate configuration.